10分钟Javascript入门


Javascript in Ten Minutes Spencer Tipping December 22, 2010 Contents 1 Introduction3 2 Types3 3 Functions4 3.1 Variadic behavior (a cool thing)...................4 3.2 Lazy scoping (a cool thing)......................4 3.3 The meaning of this (the egregious disaster)...........5 3.3.1 Important consequence: eta-reduction...........6 4 Gotchas7 4.1 Semicolon inference..........................7 4.2 Void functions.............................7 4.3 var ...................................8 4.4 Lazy scoping and mutability.....................8 4.5 Equality................................9 4.6 Boxed vs. unboxed..........................9 4.7 Things that will silently fail or misbehave............. 10 4.8 Things that will loudly fail...................... 11 4.9 Throwing things............................ 11 4.10 Be careful with typeof ........................ 11 4.11 Also be careful with instanceof .................. 12 4.12 Browser incompatibilities...................... 13 5 Prototypes 13 5.1 Why new is awful........................... 14 5.2 Why you should use prototypes................... 15 5.3 Autoboxing.............................. 15 6 A Really Awesome Equality 15 1 7 If You Have 20 Minutes... 16 7.1 Forwarding constructors....................... 17 7.2 Iterators for cool people....................... 17 7.3 Java classes and interfaces...................... 18 7.4 Recursive metaclasses........................ 19 7.5 Tail calls and delimited continuations............... 20 7.6 Syntactic macros and operator overloading............ 22 8 Further reading 24 2 1 Introduction This guide is for anyone who knows some Javascript but would like a quick1 intro to its advanced features. It will be easier reading if you also know another functional language such as Ruby, Perl, Python, ML, Scheme, etc, since I don’t really explain how first-class functions work. 2 Types Javascript has nine types. They are: 1. Null – null. Chucks a wobbly if you ask it for any attributes; e.g. null.foo fails. Never boxed.2 2. Undefined – undefined. What you get if you ask an object for something it doesn’t have; e.g. document.nonexistent. Also chucks a wobbly if you ask it for any attributes. Never boxed. 3. Strings – e.g. ’foo’,"foo" (single vs. double quotation marks makes no difference). Sometimes boxed. Instance of String when boxed. 4. Numbers – e.g. 5, 3e+10 (all numbers behave as floats – significant for division, but can be truncated by x >>> 0). Sometimes boxed. Instance of Number when boxed. 5. Booleans – true and false. Sometimes boxed. Instance of Boolean when boxed. 6. Arrays – e.g. [1, 2, "foo", [3, 4]]. Always boxed. Instance of Array. 7. Objects – e.g. {foo: ’bar’, bif: [1, 2]}, which are really just hashta- bles. Always boxed. Instance of Object. 8. Regular expressions – e.g. /foo\s*([bar]+)/. Always boxed. Instance of RegExp. 9. Functions – e.g. function (x) {return x + 1}. Always boxed. In- stance of Function. The value null is actually almost never produced by Javascript. The only case you’re likely to run across null is if you assign it somewhere (most of the time you’ll getundefined instead – one notable exception isdocument.getElementById, which returns null if it can’t find an element). Making sparing use of undefined and instead using null can make bugs much easier to track down. 1Longer than ten minutes, despite what the title says. 2Boxing is just a way of saying whether something has a pointer. A boxed type is a reference type, and an unboxed type is a value type. In Javascript, this has additional ramifications as well – see section 4.6. 3 3 Functions Functions are first-class lexical closures,3 just like lambdas in Ruby or subs in Perl.4 They behave pretty much like you’d expect, but there are several really cool things about functions and one really egregious disaster. 3.1 Variadic behavior (a cool thing) Functions are always variadic.5 Formal parameters are bound if they’re present; otherwise they’re undefined. For example: (function (x, y) {return x + y}) (’foo’) // => ’fooundefined’ The arguments to your function can be accessed in a first-class way, too: var f = function () {return arguments[0] + arguments[1]}; var g = function () {return arguments.length}; f (’foo’) // => ’fooundefined’ g (null, false, undefined) // => 3 The arguments keyword is not an array! It just looks like one. In particular, doing any of these will cause problems: arguments.concat ([1, 2, 3]) [1, 2, 3].concat (arguments) arguments.push (’foo’) arguments.shift () Toget an array from thearguments object, you can sayArray.prototype.slice.call (arguments). As far as I know that’s the best way to go about it. 3.2 Lazy scoping (a cool thing) Internally, functions use a lexical scoping chain. However, the variables inside a function body aren’t resolved until the function is called. This has some really nice advantages, perhaps foremost among them self-reference: var f = function () {return f}; f () === f // => true 3First-class in the sense that you can pass them around as values at runtime. You can’t reliably introspect them, however, because while you can obtain their source code via toString you won’t be able to access the values they close over. 4Note that block scoping isn’t used – the only scopes that get introduced are at function bound- aries. 5The number of arguments a function accepts is referred to as its arity. So a unary function, which is monadic, takes one, a binary function, which is dyadic, takes two, etc. A function that takes any number of arguments is said to be variadic. 4 Tidbit of pathology: An important consequence of lazy scoping is that you can create functions that refer to variables that might never exist. This makes Javascript very difficult to debug. The good part is that Javascript can be made to support syntactic macros via the toString method: var f = function () {return $0 + $1}; var g = eval (f.toString ().replace (/\$(\d+)/g, function (_, digits) {return ’arguments[’ + digits + ’]’})); g (5, 6) // => 11 (except on IE) Theoretically by extending this principle one could implement true structural macros, operator overloading, a type system,6 or other things. 3.3 The meaning of this (the egregious disaster) One would think it is a simple matter to figure out what this is, but it’s apparently quite challenging, and Javascript makes it look nearly impossible. Outside of functions (in the global scope, that is), the word this refers to the global object, which is window in a browser. The real question is how it behaves inside a function, and that is determined entirely by how the function is called. Here’s how that works: 1. If the function is called alone, e.g. foo(5), then inside that function’s body the word this will be equivalent to the global object. 2. If the function is called as a method, e.g. x.foo(5), then inside that function’s body the word this refers to the object, in this case x. 3. If the function starts off as a method and then is called alone: var f = x.foo; f (5); then this will be the global object again. Nothing is remembered about where f came from; it is all determined right at the invocation site. 4. If the function is invoked using apply or call, then this points to what- ever you set it to (unless you try to set it to null or undefined, in which case it will be the global object again): var f = function () {return this}; f.call (4) // => 4 f.call (0) // => 0 f.call (false) // => false f.call (null) // => [object global] 6God forbid. 5 Given this unpredictability, most Javascript libraries provide a facility to set a function’s this binding (referred to within Javascript circles as just a function’s binding) to something invocation-invariant. The easiest way to do this is to define a function that proxies arguments using apply and closes over the proper value (luckily, closure variables behave normally): var bind = function (f, this_value) { return function () {return f.apply (this_value, arguments)}; }; The difference between call andapplyis straightforward: f.call (x, y, z) is the same as f.apply (x, [y, z]), which is the same asbind (f, x) (y, z). That is, the first argument to both call and apply becomes this inside the func- tion, and the rest are passed through. In the case of apply the arguments are expected in an array-like thing (arguments works here), and in the case of call they’re passed in as given. 3.3.1 Important consequence: eta-reduction In most functional programming languages, you can eta-reduce things; that is, if you have a function of the form function (x) {return f (x)}, you can just use f instead. But in Javascript that’s not always a safe transformation; consider this code: Array.prototype.each = function (f) { for (var i = 0, l = this.length; i < l; ++i) f (this[i]); }; var xs = []; some_array.each (function (x) {xs.push (x)}); It might be tempting to rewrite it more concisely as: some_array.each (xs.push); however this latter form will result in a mysterious Javascript error when the native Array.push function finds this to be the global object instead of xs. The reason should be apparent: when the function is called inside each, it is invoked as a function instead of a method. The fact that the function started out as a method on xs is forgotten. (Just like case3 above.) The simplest way around this is to bind xs.push to xs: some_array.each (bind (xs.push, xs)); 6 4 Gotchas Javascript is an awesome language just like Perl is an awesome language and Linux is an awesome operating system. If you know how to use it properly, it will solve all of your problems trivially (well, almost), and if you miss one of its subtleties you’ll spend hours hunting down bugs. I’ve collected the things I’ve run into here, which should cover most of Javascript’s linguistic pathology.7 4.1 Semicolon inference You won’t run into any trouble if you always end lines with semicolons. How- ever, most browsers consider it to be optional, and there is one potential surprise lurking if you choose to omit them. Most of the time Javascript does what you mean. The only case where it might not is when you start a line with an open-paren, like this: var x = f (y = x) (5) Javascript joins these two lines, forming: var x = f (y = x) (5) The only way around this that I know of is to put a semicolon on the end of the first line. 4.2 Void functions Every function returns a value. If you don’t use a return statement, then your function returns undefined; otherwise it returns whatever you tell it to. This can be a common source of errors for people used to Ruby or Lisp; for instance, var x = (function (y) {y + 1}) (5); results in x being undefined. If you’re likely to make this slip, there’s an Emacs mode called “js2-mode” that identifies functions with no side-effects or return values, and it will catch most of these errors.8 7There is plenty of this pathology despite Javascript being generally an excellent language. This makes it ideal both for people who want to get things done, and for bug-connoisseurs such as myself. 8Of course, that’s if you’re an Emacs person. If you prefer a real editor (wink), I wrote a custom JS highlighter that handles some cases better than the builtin one: http://github.com/ spencertipping/js-vim-highlighter. 7 4.3 var Be careful how you define a variable. If you leave off the var keyword, your variable will be defined in the global scope, which can cause some very subtle bugs: var f = function () { // f is toplevel, so global var x = 5; // x is local to f y = 6; // y is global }; As far as I know, the same is true in both types of for loop: for (i = 0; i < 10; ++i) // i is global for (var i = 0; i < 10; ++i) // i is local to the function for (k in some_object) // k is global for (var k in some_object) // k is local to the function 4.4 Lazy scoping and mutability This is a beautiful disaster. Check this out: var x = []; for (var i = 0; i < 3; ++i) x[i] = function () { return i; }; x[0](); // What will these be? x[1](); x[2](); What will our three functions return when they are eventually called? You might expect them to return 0, 1, and 2, respectively, since those were the values of i when they were created. However they will actually all return 3. This is because of Javascript’s lazy scoping: Upon creation, each function receives only a variable name and a scope in which to search for it; the value itself is not resolved until the time of invocation, at which point i will equal 3. The simplest way to fix this is to wrap our assignment in an anonymous function that is evaluated immediately, introducing another layer of scope. The following code works because within the enclosing anonymous function, the value of new_i never changes. for (var i = 0; i < 3; ++i) (function (new_i) { x[new_i] = function () { return new_i; }; })(i); By the way, you might be tempted to do this: 8 for (var i = 0; i < 3; ++i) { var j = i; x[j] = function () { return j; }; } This won’t work for same reason that our original example failed: j will be scoped to the nearest enclosing function (remember that Javascript’s scoping is function level, not block level!), so its value is changing just as frequently as i’s. 4.5 Equality Because the == is lame, these are all true in Javascript: null == undefined null == 0 false == ’’ ’’ == 0 true == 1 true == ’1’ ’1’ == 1 So, never use the == operator unless you really want this behavior. Instead, use === (whose complement is !==), which behaves sensibly. In particular, === requires both operands to not only be the same-ish, but also be of the same type. It does referential comparison for boxed values and structural comparison for unboxed values. If one side is boxed and the other is unboxed, === will always return false. Because string literals are unboxed, though, you can use it there: ’foo’ === ’fo’ + ’o’. Tidbit of pathology: It turns out that == isn’t even stable under truthiness. If x = 0 and y = new Number(0), then x == y,!!x is false, and !!y is true. Section 4.6 talks more about why this kind of thing happens. 4.6 Boxed vs. unboxed Boxed values are always truthy and can store properties. Unboxed values will silently fail to store them; for example:9 var x = 5; x.foo = ’bar’; x.foo // => undefined; x is an unboxed number. var x = new Number (5); x.foo = ’bar’; x.foo // => ’bar’; x is a pointer. 9There are other consequences of boxing; see sections 4.10 and 4.11 for some examples. 9 How does a sometimes-boxed value acquire a box? When you do one of these things: 1. Call its constructor directly, as we did above 2. Set a member of its prototype and refer to this inside that method (see section5) All HTML objects, whether or not they’re somehow native, will be boxed. 4.7 Things that will silently fail or misbehave Javascript is very lenient about what you can get away with. In particular, the following are all perfectly legal: [1, 2, 3].foo // => undefined [1, 2, 3][4] // => undefined 1 / 0 // => Infinity 0 * ’foo’ // => NaN This can be very useful. A couple of common idioms are things like these: e.nodeType || (e = document.getElementById (e)); options.foo = options.foo || 5; Also, the language will convert anything to a string or number if you use +. All of these expressions are strings: null + [1, 2] // => ’null1,2’ undefined + [1, 2] // => ’undefined1,2’ 3 + {} // => ’3[object Object]’ ’’ + true // => ’true’ And all of these are numbers: undefined + undefined // => NaN undefined + null // => NaN null + null // => 0 {} + {} // => NaN true + true // => 2 0 + true // => 1 And some of my favorites: null * false + (true * false) + (true * true) // => 1 true << true << true // => 4 true / null // => Infinity 10 4.8 Things that will loudly fail There is a point where Javascript will complain. If you call a non-function, ask for a property of null or undefined, or refer to a global variable that doesn’t exist,10 then Javascript will throw a TypeError or ReferenceError. By extension, referring to local variables that don’t exist causes a ReferenceError, since Javascript thinks you’re talking about a global variable. 4.9 Throwing things You can throw a lot of different things, including unboxed values. This can have some advantages; in this code for instance: try { ... throw 3; } catch (n) { // n has no stack trace! } the throw/catch doesn’t compute a stack trace, making exception processing quite a bit faster than usual. But for debugging, it’s much better to throw a proper error: try { ... throw new Error(3); } catch (e) { // e has a stack trace, useful in Firebug among other things } 4.10 Be careful with typeof Because it behaves like this: typeof function () {} // => ’function’ typeof [1, 2, 3] // => ’object’ typeof {} // => ’object’ typeof null // => ’object’ typeof typeof // hangs forever in Firefox typeof is a really lame way to detect the type of something in many cases.11 Better is to use an object’s constructor property, like this: 10To get around the error for this case, you can say typeof foo, where foo is the potentially nonexistent global. It will return ’undefined’ if foo hasn’t been defined (or contains the value undefined). 11And because it returns a string, it’s marginally slower than using .constructor. 11 (function () {}).constructor // => Function [1, 2, 3].constructor // => Array ({}).constructor // => Object true.constructor // => Boolean null.constructor // TypeError: null has no properties In order to defend against null and undefined (neither of which let you ask for their constructor), you might try to rely on the falsity of these values: x && x.constructor But in fact that will fail for ’’, 0, and false. The only way I know to get around this is to just do the comparison: x === null || x === undefined ? x : x.constructor Alternatively, if you just want to find out whether something is of a given type, you can just use instanceof, which never throws an exception.12 4.11 Also be careful with instanceof instanceof is generally more useful than typeof, but it only works with boxed values. For example, these are all false: 3 instanceof Number ’foo’ instanceof String true instanceof Boolean However, these are all true: [] instanceof Array ({}) instanceof Object [] instanceof Object // Array inherits from Object /foo/ instanceof RegExp // regular expressions are always boxed (function () {}) instanceof Function One way to work around the first problem is to wrap primitives: new Number(3) instanceof Number // true new String(’foo’) instanceof String // also true new Boolean(true) instanceof Boolean // also true In general, (new x.constructor(x) instanceof x.constructor) will be true for all primitive x. However, this doesn’t hold for null or undefined. These will throw errors if you ask for their constructors, and as far as I know are never returned from the result of a constructor invocation (using new, that is). 12Well, almost. If you ask for it by putting null, undefined, or similarly inappropriate things on the right-hand side you’ll get a TypeError. 12 4.12 Browser incompatibilities Generally browsers since IE6 have good compatibility for core language stuff. One notable exception, however, is an IE bug that affects String.split: var xs = ’foo bar bif’.split (/(\s+)/); xs // on reasonable browsers: [’foo’, ’ ’, ’bar’, ’ ’, ’bif’] xs // on IE: [’foo’, ’bar’, ’bif’] A more subtle bug that took me several hours to find is that IE6 also doesn’t return functions from eval(): var f = eval(’function() {return 5}’); f() // on reasonable browsers: 5 f() // on IE6: ’Object expected’ (because f is undefined) I’m sure there are other similar bugs out there, though the most common ones to cause problems are generally in the DOM.13 5 Prototypes I used to have a very anti-OOP comment here, but considering that I occa- sionally use prototypes I removed it. Despite my obvious and probably unfair vendetta against Javascript’s linguistic compromises to pander to Java-inspired marketing pressure,14 prototype-based programming can be useful on occasion. This section contains my subjective and biased view of it. Whenever you define a function, it serves two purposes. It can be what every normal programmer assumes a function is – that is, it can take values and return values, or it can be a mutant instance-generating thing that does something completely different. Here’s an example: // A normal function: var f = function (x) {return x + 1}; f (5) // => 6 This is what most people expect. Here’s the mutant behavior that no rational person would ever imagine: // A constructor function var f = function (x) {this.x = x + 1}; // no return! var i = new f (5); // i.x = 6 The following things are true at this point: 13jQuery is your friend here. It’s branded as a Javascript library, but in fact it’s a set of enhance- ments to the DOM to (1) achieve a uniform cross-browser API, and (2) make it easier to retrieve and manipulate nodes. 14Hence its name, Javascript, despite all of the dissimilarities. 13 i.constructor === f i.__proto__ === i.constructor.prototype // on Firefox, anyway i instanceof f typeof i === ’object’ The new keyword is just a right-associative (prefix) unary operator, so you can instantiate things first-class: var x = 5; new x.constructor (); // Creates a boxed version of x, regardless of what x is new new Function(’x’, ’this.x = 5’); If you are going to program using this questionable design pattern, then you’ll probably want to add methods to things:15 var f = function (x) {this.x = x}; f.prototype.add_one = function () {++this.x}; var i = new f (5); i.add_one (); i.x // => 6 You can find tons of information about this kind of prototype programming online. 5.1 Why new is awful new has some cool features (such as being first-class), but it has a really horrible shortcoming. Most functions in Javascript can be forwarded – that is, you can write a new function to wrap an existing one and the function being called will never know the difference. For example: var to_be_wrapped = function (x) {return x + 1}; var wrapper = function () { return to_be_wrapped.apply (this, arguments); }; // for all x, wrapper(x) === to_be_wrapped(x) However, new has no such mechanism. You can’t forward a constructor in the general case, because new has no equivalent of apply. (Though this isn’t the whole story – see section 7.1 for a way hack to do it.) 15This section used to say that i.x would evaluate to 7. That isn’t true though. It’s actually 6, as indicated. (Thanks to Daniel Gasparotto for pointing this out.) 14 5.2 Why you should use prototypes If you need a dynamic-dispatch pattern, then prototypes are probably your best bet and you should use them rather than a roll-your-own approach. Google’s V8 has a bunch of prototype-specific optimizations, as do later releases of Firefox. Also, prototypes save memory; having a pointer to a prototype is much cheaper than having n pointers to n attributes. If, on the other hand, you find yourself implementing actual inheritance hierarchies, then you’re probably making a mistake.16 I have found prototypes to be an effective way to program in Javascript, but inheritance in Javascript is (1) slow,17 and (2) poorly representative of Javascript’s “everything is public” model. 5.3 Autoboxing You might be tempted to try something like this:18 Boolean.prototype.xor = function (rhs) {return !! this !== !! rhs}; And, upon running this code, you’d run into this tragically unfortunate property: false.xor (false) // => true The reason is that when you treat an unboxed value as an object (e.g. invoke one of its methods), it gets temporarily promoted into a boxed value for the purposes of that method call. This doesn’t change its value later, but it does mean that it loses whatever falsity it once had. Depending on the type you’re working with, you can convert it back to an unboxed value: function (rhs) {return !! this.valueOf () !== !! rhs}; 6 A Really Awesome Equality There is something really important about Javascript that isn’t at all obvious from the way it’s used. It is this: The syntax foo.bar is, in all situations, identical to foo[’bar’]. You could safely make this transformation to your code ahead of time, whether on value-properties, methods, or anything else. By extension, you can assign non-identifier things to object properties: 16OK, I’m being biased about this point. I tend to treat Javascript more like Scheme than like Smalltalk, so I don’t think much in terms of classical object-oriented modeling. Also, since closures are really fast, it’s OK to use functional abstraction instead of inheritance. Javascript tends to be better suited to metaprogramming than inheritance. 17In some cases really slow. The difference between single-level and multiple-level prototype lookups in Firefox 3.5, for instance, is enormous. 18!! x is just an idiom to make sure that x ends up being a boolean. It’s a double-negation, and ! always returns either true or false. 15 var foo = [1, 2, 3]; foo[’@snorkel!’] = 4; foo[’@snorkel!’] // => 4 You can also read properties this way, of course: [1, 2, 3][’length’] // => 3 [1, 2, 3][’push’] // => [native function] In fact, this is what the for (var ... in ...) syntax was built to do: Enumerate the properties of an object. So, for example: var properties = []; for (var k in document) properties.push (k); properties // => a boatload of strings However, for ... in has a dark side. It will do some very weird things when you start modifying prototypes. For example: Object.prototype.foo = ’bar’; var properties = []; for (var k in {}) properties.push (k); properties // => [’foo’] To get around this, you should do two things. First, never modify Object’s prototype, since everything is an instance of Object (including arrays and all other boxed things); and second, use hasOwnProperty:19 Object.prototype.foo = ’bar’; var properties = [], obj = {}; for (var k in obj) obj.hasOwnProperty (k) && properties.push (k); properties // => [] And very importantly, never use for ... in to iterate through arrays (it returns string indices, not numbers, which can cause problems) or strings. Either of these will fail if you add methods to Array or String (or Object, but you shouldn’t do that). 7 If You Have 20 Minutes... Javascript can do almost anything that other languages can do. However, it might not be very obvious how to go about it. 19OK, so you’re probably wondering why we don’t see the hasOwnProperty method from a for ... in loop, since it’s obviously a property. The reason is that Javascript’s attributes have invisible flags (as defined by the ECMAScript standard), one of which is called DontEnum. If DontEnum is set for some attribute, then a for ... in loop will not enumerate it. Javascript doesn’t provide a way to set the DontEnum flag on anything you add to a prototype, so using hasOwnProperty is a good way to prevent looping over other people’s prototype extensions. Note that it fails sometimes on IE6; I believe it always returns false if the prototype supplies an attribute of the same name. 16 7.1 Forwarding constructors I mentioned earlier that constructors can’t be forwarded. That isn’t exactly true. You can use this egregious hack (which is actually kind of beautiful in a don’t-try-this-at-home sort of way):20 var ctor = ...; // we want to wrap this var wrapper = function () { var params = /ˆfunction\s*\(([ˆ)]*)\)/.exec (ctor.toString()); if (params) { var unlikely = ’_’ + (Math.random() << 20).toString(36); eval (’var ’ + unlikely + ’ = ctor;’); var f = eval ( // fails on IE6 ’function (’ + params[1] + ’) {’ + ’return new ’ + unlikely + ’ (’ + params[1] + ’);’ + ’}’); return f.apply (this, arguments); } }; Now you can say wrapper(x, y, ...) and it will be translated into new ctor(x, y, ...) – and because wrapper is just a regular function, you can call its apply method to give it an array of arguments. 7.2 Iterators for cool people Because languages like Ruby showed the world just how pass´e for loops really are, a lot of self-respecting functional programmers don’t like to use them. If you’re on Firefox, you won’t have to; the Array prototype includes map and forEach functions already. But if you’re writing cross-browser code and aren’t using a library that provides them for you, here is a good way to implement them: Array.prototype.each = Array.prototype.forEach || function (f) { for (var i = 0, l = this.length; i < l; ++i) f (this[i]); return this; // convenient for chaining }; Array.prototype.map = Array.prototype.map || function (f) { var ys = []; for (var i = 0, l = this.length; i < l; ++i) ys.push (f (this[i])); return ys; }; 20In general, anytime you run into something that’s “impossible” in Javascript, you have a technique hack at your disposal. The sledgehammer is eval, which lets you do anything. 17 As far as I know this is (almost) the fastest way to write these functions. We declare two variables up-front (i and l) so that the length is cached; Javascript won’t know that this.length is invariant with the for loop, so it will check it every time if we fail to cache it. This is expensive because due to boxing we’d have a failed hash-lookup on this that then dropped down to this.__proto__, where it would find the special property length. Then, a method call would happen to retrieve length.21 The only further optimization that could be made is to go through the array backwards (which only works for each, since map is assumed to preserve order): Array.prototype.each = function (f) { for (var i = this.length - 1; i >= 0; --i) f (this[i]); }; This ends up being very slightly faster than the first implementation because it changes a floating-point subtraction (required to evaluate < for non-zero quantities) into a sign check, which internally is a bitwise and and a zero- predicated jump. Unless your Javascript engine inlines functions and you’re really determined to have killer performance (at which point I would ask why you’re using Javascript in the first place), you probably never need to consider the relative overhead of a non-zero < vs. a zero >=. You can also define an iterator for objects, but not like this: // NO NO NO!!! Don’t do it this way! Object.prototype.each = function (f) { for (var k in this) this.hasOwnProperty (k) && f (k); }; Much better is to implement a separate keys function to avoid polluting the Object prototype: var keys = function (o) { var xs = []; for (var k in o) o.hasOwnProperty (k) && xs.push (k); return xs; }; 7.3 Java classes and interfaces No sane person would ever want to use these. But if you’re insane or are being forced to, then the Google Web Toolkit will give you a way to shoot yourself in the foot and turn it into Javascript. 21This gets into how Javascript presents certain APIs. Internally it has a notion of gettable and settable properties, though there isn’t a cross-browser way to create them. But properties such as length, childNodes, etc. are all really method calls and not field lookups. (Try assigning to one and you’ll see.) 18 7.4 Recursive metaclasses There are different ways to approach this, but a straightforward way is to do something like this:22 var metaclass = {methods: { add_to: function (o) { var t = this; keys (this.methods).each (function (k) { o[k] = bind (t.methods[k], o); // can’t use /this/ here }); return o}}}; metaclass.methods.add_to.call (metaclass, metaclass); At this point, metaclass is now itself a metaclass. We can start to imple- ment instances of it: var regular_class = metaclass.add_to ({methods: {}}); regular_class.methods.def = function (name, value) { this.methods[name] = value; return this; }; regular_class.methods.init = function (o) { var instance = o || {methods: {}}; this.methods.init && this.methods.init.call (instance); return this.add_to (instance); }; regular_class.add_to (regular_class); This is a Ruby-style class where you can define public methods and a con- structor. So, for example: var point = regular_class.init (); point.def (’init’, function () {this.x = this.y = 0}); point.def (’distance’, function () { return Math.sqrt (this.x * this.x + this.y * this.y)}); We’re using the rather verbose this.x, which may offend some Python- eschewing Rubyists. Fortunately, we can use dynamic rewriting to use the $ where Rubyists would use @:23 var ruby = function (f) { return eval (f.toString ().replace (/\$(\w+)/g, function (_, name) {return ’this.’ + name})); 22Remember that a class is just a function that produces instances. Nothing about the new keyword is necessary to write object-oriented code (thank goodness). 23And, in fact, we could bake this ruby() transformation into a metaclass to make it totally transparent if we wanted to. 19 }; point.def (’init’, ruby (function () {$x = $y = 0})); point.def (’distance’, ruby (function () { return Math.sqrt ($x * $x + $y * $y)})); And now you can use that class: var p = point.init (); p.x = 3, p.y = 4; p.distance () // => 5 The advantage of using metaclasses is that you can do fun stuff with their structure. For example, suppose that we want to insert method tracing into all of our points for debugging purposes:24 keys (point.methods).each (function (k) { var original = point.methods[k]; point.methods[k] = function () { trace (’Calling method ’ + k + ’ with arguments ’ + Array.prototype.join.call (arguments, ’, ’)); return original.apply (this, arguments); }; }); Now trace (which isn’t a Javascript built-in, so you’d have to define it) would be called each time any method of a point instance was called, and it would have access to both the arguments and the state. 7.5 Tail calls and delimited continuations Javascript does not do tail-call optimization by default, which is a shame be- cause some browsers have short call stacks (the shortest I’m aware of is 500 frames, which goes by especially quickly when you have bound functions and iterators). Luckily, encoding tail calls in Javascript is actually really simple: Function.prototype.tail = function () {return [this, arguments]}; Function.prototype.call_with_tco = function () { var c = [this, arguments]; var escape = arguments[arguments.length - 1]; while (c[0] !== escape) c = c[0].apply (this, c[1]); return escape.apply (this, c[1]); }; 24The example here used to contain the expression arguments.join, which is invalid – arguments isn’t an array. Now it uses the “pretend this is an array for the purposes of calling join on it” idiom, which usually works. (Though you’ll sometimes get errors about methods not being generalized, as is the case on Chrome if you try to use Array.prototype.toString() this way.) 20 This is actually an implementation of delimited continuations, since the escape parameter serves the purpose of the final CPS target.25 So writing a factorial function, for example: // Standard recursive definition var fact1 = function (n) { return n > 0 ? n * fact1 (n - 1) : 1; }; // Tail-recursive definition var fact2 = function (n, acc) { return n > 0 ? fact2 (n - 1, acc * n) : acc; }; // With our tail-call mechanism var fact3 = function (n, acc, k) { return n > 0 ? fact3.tail (n - 1, acc * n, k) : k.tail (acc); }; The first two functions can be called normally: fact1 (5) // => 120 fact2 (5, 1) // => 120 though neither will run in constant stack space. The third one, on the other hand, will if we call it this way: var id = function (x) {return x}; fact3.call_with_tco (5, 1, id) // => 120 The way this tail-call optimization strategy works is that instead of creating new stack frames: fact1(5) 5 * fact1(4) 4 * fact1(3) ... or even creating hollow ones: fact2(5, 1) fact2(4, 5) fact2(3, 20) ... 25If this didn’t make sense, then see the Wikipedia article on continuation-passing style and delimited continuations. If that doesn’t help, then you should probably skip this section. 21 we pop out of the last stack frame before allocating a new one (treating the array of [function, args] as a kind of continuation to be returned): fact3(5, 1, k) -> [fact3, [4, 5, k]] fact3(4, 5, k) -> [fact3, [3, 20, k]] fact3(3, 20, k) ... It isn’t a bad performance hit, either – the overhead of allocating a two- element array of pointers is minimal. 7.6 Syntactic macros and operator overloading Lazy scoping lets us do some cool stuff. Let’s say we want to define a new syntax form for variable declaration, so that instead of this: var f = function () { var y = (function (x) {return x + 1}) (5); ... }; we could write this: var f = function () { var y = (x + 1).where (x = 5); ... }; This can be implemented in terms of regular expressions if we don’t mind being woefully incorrect about half the time: var expand_where = function (f) { var s = f.toString (); return eval (s.replace (/\(([ˆ)]+)\)\.where\(([ˆ)])\)/, function (_, body, value) { return ’(function (’ + value.split (’=’)[0] + ’){return ’ + body + ’}) (’ + value.split (’=’, 2)[1] + ’)’; })); }; Now we can say this: var f = expand_where (function () { var y = (x + 1).where (x = 5); ... }); Obviously a proper parser is more appropriate because it wouldn’t fail on simple paren boundaries. But the important thing is to realize that a function gives you a way to quote code, just like in Lisp: 22 (defmacro foo (bar) ...) (foo some-expression) becomes this in Javascript (assuming the existence of parse and deparse, which are rather complicated):26 var defmacro = function (transform) { return function (f) { return eval (deparse (transform (parse (f.toString ())))); }; }; var foo = defmacro (function (parse_tree) { return ...; }); foo (function () {some-expression}); This principle can be extended to allow for operator overloading if we write a transformation that rewrites operators into method calls: x << y // becomes x[’<<’](y) Remember that property names aren’t restricted to identifiers – so we could overload the << operator for arrays to work like it does in Ruby with: Array.prototype[’<<’] = function () { for (var i = 0, l = arguments.length; i < l; ++i) this.push (arguments[i]); return this; }; The only thing that’s unfortunate about implementing this stuff in Javascript rather than Lisp is that Javascript bakes syntactic constructs into the grammar, so trying to introduce new syntactic forms such as when isn’t very convenient: expand_when (function () { when (foo) { // compile error; { unexpected bar (); } }); But anything you can do inside the Javascript parse tree is fair game.27 26Real versions of these are implemented in http://github.com/spencertipping/caterwaul, if you’re interested to see what they look like. It’s also a reasonable reference for syntactic edge cases. 27Keep in mind that toString will sometimes rewrite your function to standard form, so lever- aging ambiguities of the syntax isn’t helpful. In Firefox, for example, writing expressions with excess parentheses is not useful because those excess parentheses are lost when you call toString. 23 8 Further reading I highly recommend reading jQuery (http://jquery.com) for the quality and conscientiousness of the codebase. It’s a brilliant piece of work and I’ve learned a tremendous amount by pawing around through it. Douglas Crockford has written some excellent Javascript references, includ- ing the well-known Javascript: The Good Parts and a less-well-known but free online tour of the language at http://javascript.crockford.com/survey. html.28 As a shameless plug, I also recommend reading through Divergence (http: //github.com/spencertipping/divergence), a library that I wrote. It’s very different from jQuery – much more terse and algorithmic (and has no DOM involvement). jQuery uses a more traditional approach, whereas Divergence tends to make heavy use of closures and functional metaprogramming. If you’re into Lisp and metaprogramming, you might also enjoy http:// github.com/spencertipping/divergence.rebase and http://github.com/ spencertipping/caterwaul, two projects that use function serialization and eval() to implement some of the syntactic extensions mentioned in the last section. Also, I recently found a site called http://wtfjs.com that seems to be dedicated to exposing all of Javascript’s edge-case pathologies. It’s quite a fun and enlightening read. A more in-depth look at the good, bad, and ugly parts of Javascript is http://perfectionkills.com; this site is written by one of the PrototypeJS developers and has convinced me that I really don’t know Javascript that well. 28There are some discrepancies between his view of Javascript and mine. Neither is incorrect, there are just different unstated assumptions. For example, when he says that there are three primitives he is correct; he counts types by the number of unboxed representations, whereas I count them by the number of literal constructors. 24
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